1. Field of the Invention
The present invention relates to an exhaust purification device for an internal combustion engine, and an exhaust control device and exhaust control method for an internal combustion engine.
2. Description of Related Art
Conventional exhaust purification devices for an internal combustion engines generally include two or more catalysts arranged in the exhaust system of the internal combustion engine. In such exhaust purification devices, a front catalyst is usually provided in the exhaust passage near the internal combustion engine, and a rear catalyst placed in the exhaust passage downstream of the front catalyst. The temperature of the front catalyst is quickly raised to an activation temperature, thereby making it possible to enhance the efficiency of exhaust purification at cold start of the engine. At the same time, in purifying exhaust with the front catalyst at this time, it is also important to keep the front catalyst in a state capable of purifying exhaust.
In this regard, to keep the front catalyst in a state capable of purifying exhaust, it is necessary to keep the amount of oxygen occluded in the front catalyst at an appropriate amount In this connection, there is known a technique whereby a gas sensor (for example, an A/F sensor) is arranged upstream of the front catalyst, and the output of this gas sensor is reflected on the amount of fuel injection, thus applying feedback control to the air/fuel ratio so that the air/fuel ratio of exhaust flowing into the front catalyst becomes a target air/fuel ratio (for example, the stoichometric air/fuel ratio). According to this technique, it is unlikely that the front catalyst will occlude or release an excessive amount of oxygen, thereby making it possible to keep the front catalyst in a state capable of purifying exhaust.
However, if the air/fuel ratio is feedback controlled to purify exhaust with the front catalyst as mentioned above, almost no oxygen is contained in the gas flowing out from the front catalyst. That is, the gas flowing out from the front catalyst becomes a gas whose air/fuel ratio is rich (hereinafter, “rich gas”). When the rich gas flows into the rear catalyst, stored oxygen is desorbed from the rear catalyst. Thus, continuous inflow of rich gas into the rear catalyst causes the amount of oxygen occluded in the rear catalyst to become 0 (zero), resulting in a decrease in the exhaust purification efficiency of the rear catalyst (rich poisoning).
To avoid rich poisoning the rear catalyst, a sub-feedback control of the air/fuel ratio, in addition to the above-mentioned feedback control, may be executed whereby the output of a gas sensor (for example, an oxygen sensor) provided in the exhaust passage downstream of the front catalyst is reflected on the fuel injection amount so that the air/fuel ratio of gas discharged from the front catalyst reaches a target air/fuel ratio (for example, the stoichometric air/fuel ratio). According to this technique, by executing the sub-feedback control, the air/fuel ratio of gas flowing into the rear catalyst is maintained near the stoichometric air/fuel ratio, thereby making it possible to restrain rich poisoning of the rear catalyst. However, even if the above-mentioned sub-feedback control is executed, the front catalyst must purify the exhaust gas during cold start of the engine. Thus, for the engine operation as a whole, more rich gas is likely to flow into the rear catalyst. This may also result in rich poisoning of the rear catalyst.
In order to maintain the activity of the rear catalyst in this case, gas having a lean air/fuel ratio (hereinafter, “lean gas”) must be circulated to the rear catalyst as appropriate to increase the amount of oxygen occluded in the rear catalyst. In this regard, if a bypass pipe that directs a portion of exhaust to flow into the rear catalyst while bypassing the front catalyst, and a valve that opens up and blocks up the flow path of this bypass pipe are installed, lean gas may be supplied to the rear catalyst as appropriate. It should be noted that with regard to such a bypass pipe, techniques considered as being related to the present invention are described in, for example, Japanese Patent Application Publication No. 2004-44509 (JP-A-2004-44509) and Japanese Patent Application Publication No. 11-132033 (JP-A-11-132033). Other techniques considered as being related to the present invention are described in Japanese Patent Application Publication No. 2006-283611 (JP-A-2006-283611), Japanese Patent Application Publication No. 2006-274910 (JP-A-2006-274910), Japanese Patent Application Publication No. 9-125941 (JP-A-9-125941), and Japanese Patent Application Publication No. 6-346724 (JP-A-6-346724).
FIGS. 10A and 10B are views showing a typical exhaust purification device for an internal combustion engine (hereinafter, simply referred to as exhaust purification device) 100X which includes a front catalyst and a rear catalyst, together with an internal combustion engine 50. It should be noted that in FIGS. 10A and 10B, the internal combustion engine 50 is longitudinally mounted in a vehicle (not shown), and FIGS. 10A and 10B are views of the exhaust purification device 100X as seen from above the vehicle. In the case where the internal combustion engine 50 is longitudinally mounted in the vehicle, the internal combustion engine 50 is typically subjected to large vibration in the horizontal direction as shown in the drawing. On the other hand, a flexible pipe 13 is typically arranged in an exhaust system 10X so that, when the internal combustion engine 50 vibrates, the flexible pipe 13 is deformed, thus preventing bending from occurring in the exhaust pipe or the like. In contrast, from the viewpoint of easy vehicle mounting, a bypass pipe 15X is typically arranged at a position that is horizontally spaced apart from the exhaust passage of the exhaust system 10X.
However, if the internal combustion engine 50 mainly vibrates in the horizontal direction, as described above, the arrangement of the bypass pipe 15X at a position that is horizontally spaced apart from the exhaust passage of the exhaust system 10X may result in bending deformation of the bypass pipe 15X due to the vibration of the internal combustion engine. In this case, it is difficult to absorb variations in the length of the bypass pipe 15A due to vibration, which may cause significant bending of the bypass pipe 15A, as shown in FIG. 10B. As long as such bending deformation due to vibration occurs, it is difficult to deal with the problem of rich poisoning by the provision of the bypass pipe 15X. On the other hand, the provision of the bypass pipe 15X leads to an increase in the number of parts for the vehicle as a whole, which may be disadvantageous in terms of cost. Hence, also desired is a technique that makes it possible to restrain rich poisoning by supplying lean gas to the rear catalyst 12 without providing the above-mentioned bypass pipe 15X.